taskqueued interrupt mode is going to be quite complex. Since
the polling mode is considered legacy feature for em(4) driver,
the decision is made to make polling and new interrupt handler
mutually exclusive, selected at compile time.
If kernel is compiled with DEVICE_POLLING, the fast taskqueued
interrupt handler code is disabled and the em_poll() and legacy
em_intr() functions are enabled. Otherwise, legacy functions
are disabled and only em_intr_fast() code is compiled.
Discussed with: scottl
9297f2a72e